Fuel cell separators, along with carrying out the roles of imparting electrical conductivity to each unit cell and of securing flow channels for the fuel and air (oxygen) supplied to the unit cells, also serve as boundary walls separating the unit cells. Characteristics required of a separator thus include a high electrical conductivity, a high impermeability to gases, chemical stability, heat resistance and hydrophilicity. Of these characteristics, techniques known for increasing the hydrophilicity include the methods disclosed in Patent Documents 1 to 4.
Patent Documents 1 and 2 disclose methods of hydrophilizing the surface of a fuel cell separator by treatment with fluorine gas or SO3 gas. The static contact angle of these separators is initially 20°, indicating good hydrophilicity. However, after 2,000 hours of immersion in hot water at 90° C., the static contact angle rises to 35°, Hence, a problem with these methods is that the hydrophilicity is not maintained.
Patent Document 3 discloses a method of hydrophilizing the surface of a fuel cell separator by roughening the surface, then subjecting it to atmospheric-pressure plasma treatment. The contact angle of this separator is 20° immediately after production, indicating a good hydrophilicity. However, the contact angle following one month of atmospheric storage rises to 67° and so, here too, the hydrophilicity is not maintained.
Moreover, in the fuel cell separators obtained by the methods disclosed in Patent Documents 1 to 3, the hydrophilicity decreases on account of contamination of the separator surface owing to the bleed-out of binder component low-molecular-weight ingredients and of mold release agent at the separator surface.
Patent Document 4 discloses a method of hydrophilizing the surface of a fuel cell separator by infrared laser irradiation. Because the binder component at the separator surface has been removed, there is no separator contamination due to the bleed-out of low-molecular-weight ingredients within the binder component and of mold release agent. However, the initial contact angle is 30° or more, and so the hydrophilicity is inadequate.